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Just realized the PVC pipe that I "thought" was combustion air intake for my new 90% furnace is actually the flu.
The combustion air is actually coming from a PVC 90 on the side of the furnace. Is that cool?

It's only cool if you have sufficient combustion air in your mechanical room, or a way to let air into the area that communicates with the rest of the house. The manufacturers installation instructions will tell if there is any reason not to take combustion air from inside the house.

I haven't put one in in over ten years that I didn't pipe in the combustion air.

Don't even ask why I started to do that.

It makes an immense difference in the unit operation adn the fuel efficiency, also, IF the combustion air is piped properly. This means EXACTLY like the manufacturer of the equipment outlines in the IOM.

I always direct vent 90+ furnaces preferrably with a concentric termination. Why pull air from inside the home for combustion? Where does the air come from that replaces that air? From outside through leaks in the structure. Why turn on the furnace and suck in cold air from outside around the windows, doors, electrical outlets, plumbing pipes, etc., when the air could be piped in through a dedicated intake pipe without subjecting the house to negative pressures?

There is nothing more frightful than ignorance in action....Mark Twain

Gees, not a lot of consensus here.
The unit in question is a Rheem 90 plus (RGRA model).
The manual acutally says that the combustion air source is "optional" but doesn't give a preference or make a case for either.
The dealer told me he would pipe it in if I wanted. I told him to do it. Make's sense to me.

Well you are asking a large audience. Heres what I will tell you. It depends on where the furnace is located. If its in a conditioned basement or indside the conditioned space somewhere then use two pipes regardless of room size. I mean why would you pay good money to condition air just to throw it out?

If its in the crawl space or attic its no big deal but as eddy pointed out, the pressure switch is a differential pressure switch so the resistance on both sides should be equal. I would If I were you.

Originally posted by docholiday Well you are asking a large audience. Heres what I will tell you. It depends on where the furnace is located. If its in a conditioned basement or indside the conditioned space somewhere then use two pipes regardless of room size. I mean why would you pay good money to condition air just to throw it out?

If its in the crawl space or attic its no big deal but as eddy pointed out, the pressure switch is a differential pressure switch so the resistance on both sides should be equal. I would If I were you.

HMMMMMMMMMMM.......got a call back today on 3 month old 90 unit that is in basement with one pipe system.....the pressure switch was bad,i had a spare to get them by till i get another one next week.....but you can here the switch make but no continuity on switch......i wander if this may have something to do with the switch failing.....i rarely have seen them fail....temp. switch i put on actually closes at higher point than original??????????